Network Working Group                                            D. Barr

Request for Comments: 1912             The Pennsylvania State University

Obsoletes: 1537                                            February 1996

Category: Informational

            Common DNS Operational and Configuration Errors

Status of this Memo

   This memo provides information for the Internet community.  This memo

   does not specify an Internet standard of any kind.  Distribution of

   this memo is unlimited.

Abstract

   This memo describes errors often found in both the operation of

   Domain Name System (DNS) servers, and in the data that these DNS

   servers contain.  This memo tries to summarize current Internet

   requirements as well as common practice in the operation and

   configuration of the DNS.  This memo also tries to summarize or

   expand upon issues raised in [RFC 1537].

1. Introduction

   Running a nameserver is not a trivial task.  There are many things

   that can go wrong, and many decisions have to be made about what data

   to put in the DNS and how to set up servers.  This memo attempts to

   address many of the common mistakes and pitfalls that are made in DNS

   data as well as in the operation of nameservers.  Discussions are

   also made regarding some other relevant issues such as server or

   resolver bugs, and a few political issues with respect to the

   operation of DNS on the Internet.

2. DNS Data

   This section discusses problems people typically have with the DNS

   data in their nameserver, as found in the zone data files that the

   nameserver loads into memory.

2.1 Inconsistent, Missing, or Bad Data

   Every Internet-reachable host should have a name.  The consequences

   of this are becoming more and more obvious.  Many services available

   on the Internet will not talk to you if you aren't correctly

   registered in the DNS.

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   Make sure your PTR and A records match.  For every IP address, there

   should be a matching PTR record in the in-addr.arpa domain.  If a

   host is multi-homed, (more than one IP address) make sure that all IP

   addresses have a corresponding PTR record (not just the first one).

   Failure to have matching PTR and A records can cause loss of Internet

   services similar to not being registered in the DNS at all.  Also,

   PTR records must point back to a valid A record, not a alias defined

   by a CNAME.  It is highly recommended that you use some software

   which automates this checking, or generate your DNS data from a

   database which automatically creates consistent data.

   DNS domain names consist of "labels" separated by single dots.  The

   DNS is very liberal in its rules for the allowable characters in a

   domain name.  However, if a domain name is used to name a host, it

   should follow rules restricting host names.  Further if a name is

   used for mail, it must follow the naming rules for names in mail

   addresses.

   Allowable characters in a label for a host name are only ASCII

   letters, digits, and the `-' character.  Labels may not be all

   numbers, but may have a leading digit  (e.g., 3com.com).  Labels must

   end and begin only with a letter or digit.  See [RFC 1035] and [RFC

   1123].  (Labels were initially restricted in [RFC 1035] to start with

   a letter, and some older hosts still reportedly have problems with

   the relaxation in [RFC 1123].)  Note there are some Internet

   hostnames which violate this rule (411.org, 1776.com).  The presence

   of underscores in a label is allowed in [RFC 1033], except [RFC 1033]

   is informational only and was not defining a standard.  There is at

   least one popular TCP/IP implementation which currently refuses to

   talk to hosts named with underscores in them.  It must be noted that

   the language in [1035] is such that these rules are voluntary -- they

   are there for those who wish to minimize problems.  Note that the

   rules for Internet host names also apply to hosts and addresses used

   in SMTP (See RFC 821).

   If a domain name is to be used for mail (not involving SMTP), it must

   follow the rules for mail in [RFC 822], which is actually more

   liberal than the above rules.  Labels for mail can be any ASCII

   character except "specials", control characters, and whitespace

   characters.  "Specials" are specific symbols used in the parsing of

   addresses.  They are the characters "()<>@,;:\".[]".  (The "!"

   character wasn't in [RFC 822], however it also shouldn't be used due

   to the conflict with UUCP mail as defined in RFC 976)  However, since

   today almost all names which are used for mail on the Internet are

   also names used for hostnames, one rarely sees addresses using these

   relaxed standard, but mail software should be made liberal and robust

   enough to accept them.

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   You should also be careful to not have addresses which are valid

   alternate syntaxes to the inet_ntoa() library call.  For example 0xe

   is a valid name, but if you were to type "telnet 0xe", it would try

   to connect to IP address 0.0.0.14.  It is also rumored that there

   exists some broken inet_ntoa() routines that treat an address like

   x400 as an IP address.

   Certain operating systems have limitations on the length of their own

   hostname.  While not strictly of issue to the DNS, you should be

   aware of your operating system's length limits before choosing the

   name of a host.

   Remember that many resource records (abbreviated RR) take on more

   than one argument.  HINFO requires two arguments, as does RP.  If you

   don't supply enough arguments, servers sometime return garbage for

   the missing fields.  If you need to include whitespace within any

   data, you must put the string in quotes.

2.2 SOA records

   In the SOA record of every zone, remember to fill in the e-mail

   address that will get to the person who maintains the DNS at your

   site (commonly referred to as "hostmaster").  The `@' in the e-mail

   must be replaced by a `.' first.  Do not try to put an `@' sign in

   this address.  If the local part of the address already contains a

   `.' (e.g., John.Smith@widget.xx), then you need to quote the `.' by

   preceding it with `\' character.  (e.g., to become

   John\.Smith.widget.xx) Alternately (and preferred), you can just use

   the generic name `hostmaster', and use a mail alias to redirect it to

   the appropriate persons.  There exists software which uses this field

   to automatically generate the e-mail address for the zone contact.

   This software will break if this field is improperly formatted.  It

   is imperative that this address get to one or more real persons,

   because it is often used for everything from reporting bad DNS data

   to reporting security incidents.

   Even though some BIND versions allow you to use a decimal in a serial

   number, don't.  A decimal serial number is converted to an unsigned

   32-bit integer internally anyway.  The formula for a n.m serial

   number is n*10^(3+int(0.9+log10(m))) + m which translates to

   something rather unexpected.  For example it's routinely possible

   with a decimal serial number (perhaps automatically generated by

   SCCS) to be incremented such that it is numerically larger, but after

   the above conversion yield a serial number which is LOWER than

   before.  Decimal serial numbers have been officially deprecated in

   recent BIND versions.  The recommended syntax is YYYYMMDDnn

   (YYYY=year, MM=month, DD=day, nn=revision number.  This won't

   overflow until the year 4294.

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   Choose logical values for the timer values in the SOA record (note

   values below must be expressed as seconds in the zone data):

      Refresh: How often a secondary will poll the primary server to see

          if the serial number for the zone has increased (so it knows

          to request a new copy of the data for the zone).  Set this to

          how long your secondaries can comfortably contain out-of-date

          data.  You can keep it short (20 mins to 2 hours) if you

          aren't worried about a small increase in bandwidth used, or

          longer (2-12 hours) if your Internet connection is slow or is

          started on demand.  Recent BIND versions (4.9.3) have optional

          code to automatically notify secondaries that data has

          changed, allowing you to set this TTL to a long value (one

          day, or more).

      Retry: If a secondary was unable to contact the primary at the

          last refresh, wait the retry value before trying again.  This

          value isn't as important as others, unless the secondary is on

          a distant network from the primary or the primary is more

          prone to outages.  It's typically some fraction of the refresh

          interval.

      Expire: How long a secondary will still treat its copy of the zone

          data as valid if it can't contact the primary.  This value

          should be greater than how long a major outage would typically

          last, and must be greater than the minimum and retry

          intervals, to avoid having a secondary expire the data before

          it gets a chance to get a new copy.  After a zone is expired a

          secondary will still continue to try to contact the primary,

          but it will no longer provide nameservice for the zone.  2-4

          weeks are suggested values.

      Minimum: The default TTL (time-to-live) for resource records --

          how long data will remain in other nameservers' cache.  ([RFC

          1035] defines this to be the minimum value, but servers seem

          to always implement this as the default value)  This is by far

          the most important timer.  Set this as large as is comfortable

          given how often you update your nameserver.  If you plan to

          make major changes, it's a good idea to turn this value down

          temporarily beforehand.  Then wait the previous minimum value,

          make your changes, verify their correctness, and turn this

          value back up.  1-5 days are typical values.  Remember this

          value can be overridden on individual resource records.

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   As you can see, the typical values above for the timers vary widely.

   Popular documentation like [RFC 1033] recommended a day for the

   minimum TTL, which is now considered too low except for zones with

   data that vary regularly.  Once a DNS stabilizes, values on the order

   of 3 or more days are recommended.  It is also recommended that you

   individually override the TTL on certain RRs which are often

   referenced and don't often change to have very large values (1-2

   weeks).  Good examples of this are the MX, A, and PTR records of your

   mail host(s), the NS records of your zone, and the A records of your

   nameservers.

2.3 Glue A Records

   Glue records are A records that are associated with NS records to

   provide "bootstrapping" information to the nameserver.  For example:

           podunk.xx.      in      ns      ns1.podunk.xx.

                           in      ns      ns2.podunk.xx.

           ns1.podunk.xx.  in      a       1.2.3.4

           ns2.podunk.xx.  in      a       1.2.3.5

   Here, the A records are referred to as "Glue records".

   Glue records are required only in forward zone files for nameservers

   that are located in the subdomain of the current zone that is being

   delegated.  You shouldn't have any A records in an in-addr.arpa zone

   file (unless you're using RFC 1101-style encoding of subnet masks).

   If your nameserver is multi-homed (has more than one IP address), you

   must list all of its addresses in the glue to avoid cache

   inconsistency due to differing TTL values, causing some lookups to

   not find all addresses for your nameserver.

   Some people get in the bad habit of putting in a glue record whenever

   they add an NS record "just to make sure".  Having duplicate glue

   records in your zone files just makes it harder when a nameserver

   moves to a new IP address, or is removed. You'll spend hours trying

   to figure out why random people still see the old IP address for some

   host, because someone forgot to change or remove a glue record in

   some other file.  Newer BIND versions will ignore these extra glue

   records in local zone files.

   Older BIND versions (4.8.3 and previous) have a problem where it

   inserts these extra glue records in the zone transfer data to

   secondaries.  If one of these glues is wrong, the error can be

   propagated to other nameservers.  If two nameservers are secondaries

   for other zones of each other, it's possible for one to continually

   pass old glue records back to the other.  The only way to get rid of

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   the old data is to kill both of them, remove the saved backup files,

   and restart them.  Combined with that those same versions also tend

   to become infected more easily with bogus data found in other non-

   secondary nameservers (like the root zone data).

2.4 CNAME records

   A CNAME record is not allowed to coexist with any other data.  In

   other words, if suzy.podunk.xx is an alias for sue.podunk.xx, you

   can't also have an MX record for suzy.podunk.edu, or an A record, or

   even a TXT record.  Especially do not try to combine CNAMEs and NS

   records like this!:

           podunk.xx.      IN      NS      ns1

                           IN      NS      ns2

                           IN      CNAME   mary

           mary            IN      A       1.2.3.4

   This is often attempted by inexperienced administrators as an obvious

   way to allow your domain name to also be a host.  However, DNS

   servers like BIND will see the CNAME and refuse to add any other

   resources for that name.  Since no other records are allowed to

   coexist with a CNAME, the NS entries are ignored.  Therefore all the

   hosts in the podunk.xx domain are ignored as well!

   If you want to have your domain also be a host, do the following:

           podunk.xx.      IN      NS      ns1

                           IN      NS      ns2

                           IN      A       1.2.3.4

           mary            IN      A       1.2.3.4

   Don't go overboard with CNAMEs.  Use them when renaming hosts, but

   plan to get rid of them (and inform your users).  However CNAMEs are

   useful (and encouraged) for generalized names for servers -- `ftp'

   for your ftp server, `www' for your Web server, `gopher' for your

   Gopher server, `news' for your Usenet news server, etc.

   Don't forget to delete the CNAMEs associated with a host if you

   delete the host it is an alias for.  Such "stale CNAMEs" are a waste

   of resources.

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   Don't use CNAMEs in combination with RRs which point to other names

   like MX, CNAME, PTR and NS.  (PTR is an exception if you want to

   implement classless in-addr delegation.)  For example, this is

   strongly discouraged:

           podunk.xx.      IN      MX      mailhost

           mailhost        IN      CNAME   mary

           mary            IN      A       1.2.3.4

   [RFC 1034] in section 3.6.2 says this should not be done, and [RFC

   974] explicitly states that MX records shall not point to an alias

   defined by a CNAME.  This results in unnecessary indirection in

   accessing the data, and DNS resolvers and servers need to work more

   to get the answer.  If you really want to do this, you can accomplish

   the same thing by using a preprocessor such as m4 on your host files.

   Also, having chained records such as CNAMEs pointing to CNAMEs may

   make administration issues easier, but is known to tickle bugs in

   some resolvers that fail to check loops correctly.  As a result some

   hosts may not be able to resolve such names.

   Having NS records pointing to a CNAME is bad and may conflict badly

   with current BIND servers.  In fact, current BIND implementations

   will ignore such records, possibly leading to a lame delegation.

   There is a certain amount of security checking done in BIND to

   prevent spoofing DNS NS records.  Also, older BIND servers reportedly

   will get caught in an infinite query loop trying to figure out the

   address for the aliased nameserver, causing a continuous stream of

   DNS requests to be sent.

2.5 MX records

   It is a good idea to give every host an MX record, even if it points

   to itself!  Some mailers will cache MX records, but will always need

   to check for an MX before sending mail.  If a site does not have an

   MX, then every piece of mail may result in one more resolver query,

   since the answer to the MX query often also contains the IP addresses

   of the MX hosts.  Internet SMTP mailers are required by [RFC 1123] to

   support the MX mechanism.

   Put MX records even on hosts that aren't intended to send or receive

   e-mail.  If there is a security problem involving one of these hosts,

   some people will mistakenly send mail to postmaster or root at the

   site without checking first to see if it is a "real" host or just a

   terminal or personal computer that's not set up to accept e-mail.  If

   you give it an MX record, then the e-mail can be redirected to a real

   person.  Otherwise mail can just sit in a queue for hours or days

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   until the mailer gives up trying to send it.

   Don't forget that whenever you add an MX record, you need to inform

   the target mailer if it is to treat the first host as "local".  (The

   "Cw" flag in sendmail, for example)

   If you add an MX record which points to an external host (e.g., for

   the purposes of backup mail routing) be sure to ask permission from

   that site first.  Otherwise that site could get rather upset and take

   action (like throw your mail away, or appeal to higher authorities

   like your parent DNS administrator or network provider.)

2.6 Other Resource Records

2.6.1 WKS

   WKS records are deprecated in [RFC 1123].  They serve no known useful

   function, except internally among LISP machines.  Don't use them.

2.6.2 HINFO

   On the issue HINFO records, some will argue that these is a security

   problem (by broadcasting what vendor hardware and operating system

   you so people can run systematic attacks on known vendor security

   holes).  If you do use them, you should keep up to date with known

   vendor security problems.  However, they serve a useful purpose.

   Don't forget that HINFO requires two arguments, the hardware type,

   and the operating system.

   HINFO is sometimes abused to provide other information.  The record

   is meant to provide specific information about the machine itself.

   If you need to express other information about the host in the DNS,

   use TXT.

2.6.3 TXT

   TXT records have no specific definition.  You can put most anything

   in them.  Some use it for a generic description of the host, some put

   specific information like its location, primary user, or maybe even a

   phone number.

2.6.4 RP

   RP records are relatively new.  They are used to specify an e-mail

   address (see first paragraph of section 2.2)  of the "Responsible

   Person" of the host, and the name of a TXT record where you can get

   more information.  See [RFC 1183].

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2.7 Wildcard records

   Wildcard MXs are useful mostly for non IP-connected sites.  A common

   mistake is thinking that a wildcard MX for a zone will apply to all

   hosts in the zone.  A wildcard MX will apply only to names in the

   zone which aren't listed in the DNS at all.  e.g.,

           podunk.xx.      IN      NS      ns1

                           IN      NS      ns2

           mary            IN      A       1.2.3.4

           *.podunk.xx.    IN      MX      5 sue

   Mail for mary.podunk.xx will be sent to itself for delivery.  Only

   mail for jane.podunk.xx or any hosts you don't see above will be sent

   to the MX.  For most Internet sites, wildcard MX records are not

   useful.  You need to put explicit MX records on every host.

   Wildcard MXs can be bad, because they make some operations succeed

   when they should fail instead.  Consider the case where someone in

   the domain "widget.com" tries to send mail to "joe@larry".  If the

   host "larry" doesn't actually exist, the mail should in fact bounce

   immediately.  But because of domain searching the address gets

   resolved to "larry.widget.com", and because of the wildcard MX this

   is a valid address according to DNS.  Or perhaps someone simply made

   a typo in the hostname portion of the address.  The mail message then

   gets routed to the mail host, which then rejects the mail with

   strange error messages like "I refuse to talk to myself" or "Local

   configuration error".

   Wildcard MX records are good for when you have a large number of

   hosts which are not directly Internet-connected (for example, behind

   a firewall) and for administrative or political reasons it is too

   difficult to have individual MX records for every host, or to force

   all e-mail addresses to be "hidden" behind one or more domain names.

   In that case, you must divide your DNS into two parts, an internal

   DNS, and an external DNS.  The external DNS will have only a few

   hosts and explicit MX records, and one or more wildcard MXs for each

   internal domain.  Internally the DNS will be complete, with all

   explicit MX records and no wildcards.

   Wildcard As and CNAMEs are possible too, and are really confusing to

   users, and a potential nightmare if used without thinking first.  It

   could result (due again to domain searching) in any telnet/ftp

   attempts from within the domain to unknown hosts to be directed to

   one address.  One such wildcard CNAME (in *.edu.com) caused

   Internet-wide loss of services and potential security nightmares due

   to unexpected interactions with domain searching.  It resulted in

   swift fixes, and even an RFC ([RFC 1535]) documenting the problem.

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2.8 Authority and Delegation Errors (NS records)

   You are required to have at least two nameservers for every domain,

   though more is preferred.  Have secondaries outside your network.  If

   the secondary isn't under your control, periodically check up on them

   and make sure they're getting current zone data from you.  Queries to

   their nameserver about your hosts should always result in an

   "authoritative" response.  If not, this is called a "lame

   delegation".  A lame delegations exists when a nameserver is

   delegated responsibility for providing nameservice for a zone (via NS

   records) but is not performing nameservice for that zone (usually

   because it is not set up as a primary or secondary for the zone).

   The "classic" lame delegation can be illustrated in this example:

           podunk.xx.      IN      NS      ns1.podunk.xx.

                           IN      NS      ns0.widget.com.

   "podunk.xx" is a new domain which has recently been created, and

   "ns1.podunk.xx" has been set up to perform nameservice for the zone.

   They haven't quite finished everything yet and haven't made sure that

   the hostmaster at "ns0.widget.com" has set up to be a proper

   secondary, and thus has no information about the podunk.xx domain,

   even though the DNS says it is supposed to.  Various things can

   happen depending on which nameserver is used.  At best, extra DNS

   traffic will result from a lame delegation.  At worst, you can get

   unresolved hosts and bounced e-mail.

   Also, sometimes a nameserver is moved to another host or removed from

   the list of secondaries.  Unfortunately due to caching of NS records,

   many sites will still think that a host is a secondary after that

   host has stopped providing nameservice.  In order to prevent lame

   delegations while the cache is being aged, continue to provide

   nameservice on the old nameserver for the length of the maximum of

   the minimum plus refresh times for the zone and the parent zone.

   (See section 2.2)

   Whenever a primary or secondary is removed or changed, it takes a

   fair amount of human coordination among the parties involved.  (The

   site itself, it's parent, and the site hosting the secondary)  When a

   primary moves, make sure all secondaries have their named.boot files

   updated and their servers reloaded.  When a secondary moves, make

   sure the address records at both the primary and parent level are

   changed.

   It's also been reported that some distant sites like to pick popular

   nameservers like "ns.uu.net" and just add it to their list of NS

   records in hopes that they will magically perform additional

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   nameservice for them.  This is an even worse form of lame delegation,

   since this adds traffic to an already busy nameserver.  Please

   contact the hostmasters of sites which have lame delegations.

   Various tools can be used to detect or actively find lame

   delegations.  See the list of contributed software in the BIND

   distribution.

   Make sure your parent domain has the same NS records for your zone as

   you do.  (Don't forget your in-addr.arpa zones too!).  Do not list

   too many (7 is the recommended maximum), as this just makes things

   harder to manage and is only really necessary for very popular top-

   level or root zones.  You also run the risk of overflowing the 512-

   byte limit of a UDP packet in the response to an NS query.  If this

   happens, resolvers will "fall back" to using TCP requests, resulting

   in increased load on your nameserver.

   It's important when picking geographic locations for secondary

   nameservers to minimize latency as well as increase reliability.

   Keep in mind network topologies.  For example if your site is on the

   other end of a slow local or international link, consider a secondary

   on the other side of the link to decrease average latency.  Contact

   your Internet service provider or parent domain contact for more

   information about secondaries which may be available to you.

3. BIND operation

   This section discusses common problems people have in the actual

   operation of the nameserver (specifically, BIND).  Not only must the

   data be correct as explained above, but the nameserver must be

   operated correctly for the data to be made available.

3.1 Serial numbers

   Each zone has a serial number associated with it.  Its use is for

   keeping track of who has the most current data.  If and only if the

   primary's serial number of the zone is greater will the secondary ask

   the primary for a copy of the new zone data (see special case below).

   Don't forget to change the serial number when you change data!  If

   you don't, your secondaries will not transfer the new zone

   information.  Automating the incrementing of the serial number with

   software is also a good idea.

   If you make a mistake and increment the serial number too high, and

   you want to reset the serial number to a lower value, use the

   following procedure:

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      Take the `incorrect' serial number and add 2147483647 to it.  If

      the number exceeds 4294967296, subtract 4294967296.  Load the

      resulting number.  Then wait 2 refresh periods to allow the zone

      to propagate to all servers.

      Repeat above until the resulting serial number is less than the

      target serial number.

      Up the serial number to the target serial number.

   This procedure won't work if one of your secondaries is running an

   old version of BIND (4.8.3 or earlier).  In this case you'll have to

   contact the hostmaster for that secondary and have them kill the

   secondary servers, remove the saved backup file, and restart the

   server.  Be careful when editing the serial number -- DNS admins

   don't like to kill and restart nameservers because you lose all that

   cached data.

3.2 Zone file style guide

   Here are some useful tips in structuring your zone files.  Following

   these will help you spot mistakes, and avoid making more.

   Be consistent with the style of entries in your DNS files. If your

   $ORIGIN is podunk.xx., try not to write entries like:

           mary            IN      A       1.2.3.1

           sue.podunk.xx.  IN      A       1.2.3.2

   or:

           bobbi           IN      A       1.2.3.2

                           IN      MX      mary.podunk.xx.

   Either use all FQDNs (Fully Qualified Domain Names) everywhere or

   used unqualified names everywhere.  Or have FQDNs all on the right-

   hand side but unqualified names on the left.  Above all, be

   consistent.

   Use tabs between fields, and try to keep columns lined up.  It makes

   it easier to spot missing fields (note some fields such as "IN" are

   inherited from the previous record and may be left out in certain

   circumstances.)

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   Remember you don't need to repeat the name of the host when you are

   defining multiple records for one host.  Be sure also to keep all

   records associated with a host together in the file.  It will make

   things more straightforward when it comes time to remove or rename a

   host.

   Always remember your $ORIGIN.  If you don't put a `.' at the end of

   an FQDN, it's not recognized as an FQDN.  If it is not an FQDN, then

   the nameserver will append $ORIGIN to the name.  Double check, triple

   check, those trailing dots, especially in in-addr.arpa zone files,

   where they are needed the most.

   Be careful with the syntax of the SOA and WKS records (the records

   which use parentheses).  BIND is not very flexible in how it parses

   these records.  See the documentation for BIND.

3.3 Verifying data

   Verify the data you just entered or changed by querying the resolver

   with dig (or your favorite DNS tool, many are included in the BIND

   distribution) after a change.  A few seconds spent double checking

   can save hours of trouble, lost mail, and general headaches.  Also be

   sure to check syslog output when you reload the nameserver.  If you

   have grievous errors in your DNS data or boot file, named will report

   it via syslog.

   It is also highly recommended that you automate this checking, either

   with software which runs sanity checks on the data files before they

   are loaded into the nameserver, or with software which checks the

   data already loaded in the nameserver.  Some contributed software to

   do this is included in the BIND distribution.

4. Miscellaneous Topics

4.1 Boot file setup

   Certain zones should always be present in nameserver configurations:

           primary         localhost               localhost

           primary         0.0.127.in-addr.arpa    127.0

           primary         255.in-addr.arpa        255

           primary         0.in-addr.arpa          0

   These are set up to either provide nameservice for "special"

   addresses, or to help eliminate accidental queries for broadcast or

   local address to be sent off to the root nameservers.  All of these

   files will contain NS and SOA records just like the other zone files

   you maintain, the exception being that you can probably make the SOA

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   timers very long, since this data will never change.

   The "localhost" address is a "special" address which always refers to

   the local host.  It should contain the following line:

           localhost.      IN      A       127.0.0.1

   The "127.0" file should contain the line:

           1    PTR     localhost.

   There has been some extensive discussion about whether or not to

   append the local domain to it.  The conclusion is that "localhost."

   would be the best solution.  The reasons given include:

      "localhost" by itself is used and expected to work in some

      systems.

      Translating 127.0.0.1 into "localhost.dom.ain" can cause some

      software to connect back to the loopback interface when it didn't

      want to because "localhost" is not equal to "localhost.dom.ain".

   The "255" and "0" files should not contain any additional data beyond

   the NS and SOA records.

   Note that future BIND versions may include all or some of this data

   automatically without additional configuration.

4.2 Other Resolver and Server bugs

   Very old versions of the DNS resolver have a bug that cause queries

   for names that look like IP addresses to go out, because the user

   supplied an IP address and the software didn't realize that it didn't

   need to be resolved.  This has been fixed but occasionally it still

   pops up.  It's important because this bug means that these queries

   will be sent directly to the root nameservers, adding to an already

   heavy DNS load.

   While running a secondary nameserver off another secondary nameserver

   is possible, it is not recommended unless necessary due to network

   topologies.  There are known cases where it has led to problems like

   bogus TTL values.  While this may be caused by older or flawed DNS

   implementations, you should not chain secondaries off of one another

   since this builds up additional reliability dependencies as well as

   adds additional delays in updates of new zone data.

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4.3 Server issues

   DNS operates primarily via UDP (User Datagram Protocol) messages.

   Some UNIX operating systems, in an effort to save CPU cycles, run

   with UDP checksums turned off.  The relative merits of this have long

   been debated.  However, with the increase in CPU speeds, the

   performance considerations become less and less important.  It is

   strongly encouraged that you turn on UDP checksumming to avoid

   corrupted data not only with DNS but with other services that use UDP

   (like NFS).  Check with your operating system documentation to verify

   that UDP checksumming is enabled.

References

   [RFC 974] Partridge, C., "Mail routing and the domain system", STD

              14, RFC 974, CSNET CIC BBN Laboratories Inc, January 1986.

   [RFC 1033] Lottor, M, "Domain Administrators Operations Guide", RFC

              1033, USC/Information Sciences Institute, November 1987.

   [RFC 1034] Mockapetris, P., "Domain Names - Concepts and Facilities",

              STD 13, RFC 1034, USC/Information Sciences Institute,

              November 1987.

   [RFC 1035] Mockapetris, P., "Domain Names - Implementation and

              Specification", STD 13, RFC 1035, USC/Information Sciences

              Institute, November 1987.

   [RFC 1123] Braden, R., "Requirements for Internet Hosts --

              Application and Support", STD 3, RFC 1123, IETF, October

              1989.

   [RFC 1178] Libes, D., "Choosing a Name for Your Computer", FYI 5, RFC

              1178, Integrated Systems Group/NIST, August 1990.

   [RFC 1183] Ullman, R., Mockapetris, P., Mamakos, L, and C. Everhart,

              "New DNS RR Definitions", RFC 1183, October 1990.

   [RFC 1535] Gavron, E., "A Security Problem and Proposed Correction

              With Widely Deployed DNS Software", RFC 1535, ACES

              Research Inc., October 1993.

   [RFC 1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.

              Miller, "Common DNS Implementation Errors and Suggested

              Fixes", RFC 1536, USC/Information Sciences Institute, USC,

              October 1993.

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RFC 1912                   Common DNS Errors               February 1996

   [RFC 1537] Beertema, P., "Common DNS Data File Configuration Errors",

              RFC 1537, CWI, October 1993.

   [RFC 1713] A. Romao, "Tools for DNS debugging", RFC 1713, FCCN,

              November 1994.

   [BOG] Vixie, P, et. al., "Name Server Operations Guide for BIND",

              Vixie Enterprises, July 1994.

5. Security Considerations

   Security issues are not discussed in this memo.

6. Author's Address

   David Barr

   The Pennsylvania State University

   Department of Mathematics

   334 Whitmore Building

   University Park, PA 16802

   Voice: +1 814 863 7374

   Fax: +1 814 863-8311

   EMail: barr@math.psu.edu